Cymbalta and Wellbutrin are not in the same class of drugs, no. The following information comes from
www.rxlist.com, a reliable source of information I've used often.
Cymbaltaź (duloxetine hydrochloride) is a selective serotonin and norepinephrine reuptake inhibitor (SSNRI).
WELLBUTRIN (bupropion hydrochloride), an antidepressant of the aminoketone class, is chemically unrelated to tricyclic, tetracyclic, selective serotonin re-uptake inhibitor, or other known antidepressant agents.
Both of these meds are for depression.
Seroquel is used for both depression and mania in bipolar I disorder, and for maintenance when used with lithium or depakote.
It is not unusual for people with bipolar spectrum disorder to be on a cocktail of meds. It's often a matter of fine tuning; using one med to offset complications of another med which may be necessary to control moods, etc. Please talk to the doctor about this, as the doc is the only one who can fully explain the rational for your son's particular combination of meds. I only know that it is not unusual at all.
I hope this helps. Others will come and share their ESH.
Shalom!